According to prior practice, a typical furnace is configured for only one mode of operation (upflow or downflow), depending upon the configuration of the existing ductwork with which the furnace is installed. For example, if the ductwork is located above the space in which the furnace is installed, the furnace must be configured for upflow operation, whereby air is blown upwardly. Conversely, if the existing ductwork is located below the space in which the furnace is installed, the furnace must be configured for downflow operation, whereby air is blown downwardly. If the existing ductwork is located laterally with respect to the space in which the furnace is installed, the furnace must be positioned horizontally, whereby air is blown horizontally into the supply air duct. Because of the different ductwork configurations, an installer may have to include in his inventory different types of furnaces (e.g., upflow, downflow and horizontal flow)in order to meet anticipated demand.
In a combustion furnace used for space heating, fuel, such as natural gas, is burned in one or more burners and the products of combustion are drawn through a heat exchanger by a combustion gas blower. An elongated flue is located on the discharge side of the combustion gas blower for exhausting products of combustion from the cabinet in which the furnace components are housed. According to prior practice, conversion of a typical combustion furnace from upflow to downflow operation, or vice-versa, involves reversing the respective positions of the burner, heat exchanger and combustion gas blower. It also involves relocation of the furnace control panel on which the electrical components of the furnace are mounted and modification of the compartment in which an air blower (either forced or induced draft) is located, to accommodate passage of the exhaust flue. An airtight seal must be applied between the exhaust flue and the air blower compartment, to prevent supply air from mixing with products of combustion. Obviously, conversion of the typical combustion furnace between upflow and downflow operation must be done in the factory and cannot be readily accomplished in the field.
One type of prior art furnace is field convertible between upflow and downflow operation. In this type of furnace, only the combustion gas blower is reversed by demounting the combustion gas blower and flue plate from a flue box in which products of combustion emanating from the heat exchanger accumulate and remounting the combustion gas blower and flue plate in an opposite position, wherein the flue extends in an opposite direction. Although this type of furnace is convertible between upflow and downflow operation, the reversing procedure may damage gaskets on the flue box and flue plate, which can result in leakage of products of combustion. Further, when the furnace is configured for downflow operation, the flue extends past the burners and blocks access thereto, which makes servicing the burners more difficult. The heat exchanger is not reversible on this type of furnace. There is, therefore, a need for a furnace which is field convertible for upflow or downflow operation, depending upon the configuration of the existing ductwork with which the furnace is to be installed.
Another problem associated with prior art combustion furnaces is the efficiency penalty associated with laminar flow of combustion gases through heat exchanger tubes. Although laminar flow is desirable for effective venting of combustion gases, it is detrimental to furnace efficiency because substantial heat is lost through the flue. One prior art attempt to improve furnace efficiency involves placing baffles at the discharge end of the heat exchanger. Although this configuration is effective in slowing down the combustion gases near the discharge end of the heat exchanger, it is not effective in slowing down the combustion gases throughout the entire length of the heat exchanger tubes. It is not feasible to baffle the entire length of the heat exchanger tubes for mass production. There is, therefore, a need for a heat exchanger with improved efficiency, which is suitable for mass production.
Yet another problem associated with prior art furnaces is the problem of accurately measuring the temperature of the air blown across the heat exchanger (i.e., the supply air stream). Typically, a temperature sensitive bi-metallic disk is positioned for measuring the temperature of the supply air stream. If the temperature becomes abnormally high, the furnace must be automatically shut down and cool down procedures initiated.
Typically, the temperature sensing element is mounted on the ends of two elongated posts, which extend from the furnace vestibule panel into the supply air stream. Because of the non-rigidity of the mounting posts, it is difficult to accurately position the sensing element within the air stream. There is, therefore, a need for improved mounting apparatus for mounting a temperature sensing element for measuring the temperature of a furnace supply air stream.